Known in the art is a method for preparing .alpha.-naphthol by sulphonation of naphthalene with a 96-98% sulphuric acid taken in a 1.5-2-fold excess at a temperature ranging from 20.degree. to 75.degree. C. Then the resulting sulpho mass consisting of naphthalenesulphonic acids, unreacted sulphuric acid, water and impurities (such as sulphones, disulphonic acids) is diluted with water in a 3.5-fold excess. To separate the unreacted sulphuric acid, the aqueous solution of the sulpho mass is treated with lime and the precipitated CaSO.sub.4 is separated therefrom. Thereafter, the resulting calcium salt of naphthalenesulphonic acid in the form of an aqueous solution is converted, by means of Na.sub.2 CO.sub.3, to a sodium salt of naphthalenesulphonic acid. Afterwards, the thus-prepared aqueous solution of sodium salt of naphthalenesulphonic acid is treated with hydrogen chloride to neutralize the excess soda. The resulting mixture is evaporated and dried. As a result, after drying a product is obtained having the following composition, percent by weight:
______________________________________ sodium salt of naphthalene-.beta.-sulphonic acid 10.2 sodium salt of naphthalene-.alpha.-sulphonic acid 77.5 disulphonate 5.7 sodium sulphate 2.9 water 3.7 ______________________________________
As is clearly seen from the data, the resulting sodium salt of naphthalene-.alpha.-sulphonic acid is diluted with .beta.-isomer and disulphonate.
A mixture of naphtholates and an alkali metal sulphite contaminated with an alkali metal sulphate and resinous compounds is obtained by melt fusion of sodium salts of naphthalenesulphonic acids with potassium hydroxide or sodium hydroxide at a temperature of 290.degree. C. for 8 hours.
Said mixture of the fusion products is diluted with water taken in a great excess and acidified with sulphuric or hydrochloric acid at a temperature ranging from 70.degree. to 95.degree. C. Mineral acids are also taken in an excess of 2-4 moles per mole of the naphtholate.
Then the resulting mixture in the form of an aqueous solution consisting of naphthols, and alkali metal sulphate or chloride, the unreacted sulphite, resinous compounds and other impurities is cooled to room temperature. In doing so, a mixture of 89% by weight of .alpha.-naphthol and 11% by weight of .beta.-naphthol is precipitated from the solution.
The yield of .alpha.-naphthol is 79 mol.% as calculated for the starting naphthalene. The resulting .alpha.-naphthol contains more than 10% by weight of .beta.-naphthol which makes it unsuitable in the production of .alpha.-naphthyl ether of N-methylcarbamine acid or in the synthesis of .alpha.-naphthylamine containing no strongly carcinogenic .beta.-naphthylamine.
Therefore, this prior art (prototype) method has the following disadvantages:
low yield and purity of .alpha.-naphthol,
a high rate of consumption of the reactants,
the formation of hardly-utilized wastes: gypsum, acidic waters, disulphonates and resinous products.
In addition, a disadvantage of the prior art method resides in the complicated process equipment employed.
Due to the above-mentioned disadvantages the prior art method discussed hereinabove is not suitable for the preparation of a high-purity .alpha.-naphthol.